This invention relates to a multi-element radiation detector commonly used in an X-ray computed tomography for medical use and the like, and more particularly to an X-ray detector of the above type in which undesirable crosstalk between the adjacent X-ray detection elements can be prevented thereby ensuring the desired uniform spectral sensitivity while improving the industrial productivity.
A radiation detector is now widely used in an X-ray computed tomography for medical use, an X-ray scanography system or the like and is also used in a baggage inspection system or the like, and an improvement in the detection operation performance of the radiation detector is more and more demanded so as to improve the quality of reconstructed images displayed as a result of detection by the detector. Among the apparatuses described above, the Xenon ionization detector is commonly used as the X-ray detector in the X-ray computed tomography. This conventional X-ray detector is being gradually replaced now by a multi-element radiation solid-state detector (referred to hereinafter merely as a detector) capable of operating with a higher S/N ratio. FIG. 4 is a schematic perspective view showing the basic structure of the detector described above.
Referring to FIG. 4, the detector includes scintillators 1 converting incident X-ray 5 into visible light, a plurality of isolation plates 2 isolating the adjacent X-ray detection elements from each other, and a multi-channels Si photodiode 3 formed on a substrate 4 for converting the visible light emitted from the scintillators 1 into an electrical signal. The scintillators 1 are bonded to an upper surface 3a of the Si photodiode 3, and those belonging to the individual channels of the Si photodiode 3 are arranged in parallel to each other with a predetermined pitch on the substrate 4 together with the isolation plates 2 thereby forming an array of the X-ray detection elements. A plurality of light detection parts 3b are formed on the surface 3a of the Si photodiode 3 as shown.
In the detector having the structure shown in FIG. 4, the scintillators 1 convert the incident X-ray 5 into visible light having an intensity proportional to the intensity of the incident X-ray 5. While being repeatedly reflected by the surfaces of the isolation plates 2, the interfaces or surfaces of the scintillators 1, etc., the visible light produced by conversion is guided toward the light detection parts 3b of the Si photodiode 3. The visible light is subjected to photoelectric conversion at the light detection parts 3b to be detected as an electrical signal (photocurrent) having the strength proportional to the intensity of the light supplied from the scintillators 1.
It is well known that whether the optical characteristic of such a detecter is satisfactory or not depends mainly upon the S/N ratio as well as the spatial resolution, and the S/N ratio is determined by the efficiency of conversion of the incident X-ray 5 into its output signal, that is, the quantum efficiency. In order to improve this quantum efficiency, it is necessary to improve the light collection efficiency of the scintillators 1, to improve the photoelectric conversion efficiency of the Si photodiode 3, to improve the X-ray spatial efficiency that is the X-ray quantum efficiency of the detector, and also to improve the light transmission efficiency inside the detector.
Among the efficiencies described above, the quantum efficiency can be improved by minimizing the area of the region, such as, the space which is occupied by the individual isolation plates 2 and does not contribute to the detection of the incident X-ray 5, that is, the region except that occupied by the scintillators 1. Further, the light transmission efficiency inside the detector can be improved by minimizing both the absorption of light in the scintillators 1 themselves and the absorption of light by the surfaces of the isolation plates 2, so that the light emitted from the scintillators 1 can be efficiently guided toward the Si photodiode 3.
On the other hand, the spatial resolution of a reconstructed image in the X-ray computed tomography using the detector depends on the distance between the adjacent isolation plates 2 shown in FIG. 4, that is, the width of each of the X-ray detection elements forming the detector, and it is the recent tendency that this width of each X-ray detection element is selected to be less than 1 mm in many cases. When the width of each X-ray detection element is reduced as described above, the quantity of the X-ray 5 incident on each of the detection elements decreases to a level lower than the level of the output signal from the detection elements, resulting in an undesirable decrease of the S/N ratio. In addition, the light emitted from each of the scintillators 1 tends to be partly absorbed by the scintillator 1 itself and tends also to be repeatedly reflected in various directions by the surfaces of the scintillator 1 and the associated isolation plate 2 and also at the both ends 21 of the scintillator 1 located in the direction of cutting, etc., with the result that the proportion of direct arrival of the light incident on the Si photodiode 3 is decreased to lower the light transmission efficiency described above. In this case, the light reflectivity of each detection element especially at the both ends 21 of each scintillator 1 located in the direction of cutting cannot be made constant, with the result that undesirable crosstalk tends to occur between the adjacent detection elements. Thus, it is inevitable that the detection elements have optical characteristics different from each other.
Therefore, the individual detection elements have respectively different spectral sensitivities. As a result, in an X-ray computed tomography of third generation that employs a rotate-rotate method according to which a radiation source and a radiation detector rotate together, a ring artifact tends to appear on a reconstructed image.
An idea for controlling the reflection of light at the both ends 21 of each scintillator 1 located in the direction of cutting is proposed in, for example, JP-A-1-191087. The disclosure of the application includes the steps of polishing the surfaces of the scintillator 1 into the state of specular reflection, coating a thin transparent film of a material, such as, a resin on the five surfaces except the surface engaged by an associated light detection element, and forming a light reflective film on the surface of the resin film by vacuum evaporation or like means, for the purpose of maintaining constant the state of light reflection and improving the efficiency of light reflection. Thus, the light emitted from the scintillator 1 can be concentrated on the surface engaged by the associated light detection element so as to effectively utilize the light.
In the case of the structure disclosed in JP-A-1-191087 cited above, effective utilization of light and suppression of crosstalk in one light detection element can be attained However, it is technically difficult to ensure that the same surface state can be satisfactorily optically reproducibly maintained for the individual scintillators 1 at all times. Especially, it is technically extremely difficult to form the light reflective film, under the same condition and simultaneously, on the five surfaces of each scintillator 1 except the surface engaged by the associated light detection element. Thus, the light reflective film is practically separately formed on each of the individual scintillators 1, with the result that an undesirable fluctuation occurs inevitably between the optical characteristics of the individual light detection elements.
Further, the resin or like light reflective film formed on the five surfaces of each scintillator 1 except the surface engaged by the associated light detection element is generally inferior in its radiation durability to inorganic materials and tends to be peeled off or discolored when it is exposed to radiation for a long time. Thus, such a problem has been commonly encountered in which an undesirable ring artifact tends to appear due to the tendency of occurrence of fluctuation of the optical characteristics between the light detection elements with lapse of time.
Further, because of the fact that a modern X-ray computed tomography uses a multi-element radiation detector including as many as 1000 light detection elements, it is difficult to form all the light detection elements so that they have the same optical characteristics, and, because of an inevitable fluctuation of their optical characteristics, appearance of an undesirable ring artifact cannot be avoided. Also, many optical processing steps are required for the formation of the light detection elements. Thus, such another problem arises that the industrial productivity of the detector is low.
Furthermore, because the individual scintillators are separately manufactured as described above, it is necessary that the separately manufactured scintillators are to be precisely arranged on the Si photodiode during the process of assembling the detector. However, it is quite difficult to precisely and uniformly arrange so many scintillators on the Si photodiode, with the result that not only the industrial productivity is lowered, but also non-uniform precision leads to a fluctuation of the optical characteristics of the light detection elements. Thus, this leads to such another problem that the quality of a reconstructed image is inevitably degraded.